Some embodiments relate to a method and a system which makes it possible in particular to prepare polymer/reduced graphene oxide nanocomposites by in situ reduction of graphene oxide, by using fluids under supercritical and subcritical conditions. It applies generally for the in situ reduction of graphene oxide present in a polymer component in order to obtain a material exhibiting good properties, in particular of electrical conductivity.
Graphene is a material which is difficult to incorporate in polymer matrices. One approach commonly employed for overcoming this difficulty consists in oxidizing graphite in order to form monolayer graphene oxide. Graphene oxide GO, in contrast to graphene, can easily be handled and dissolved in various polar solvents. This ready dissolution of graphene oxide GO makes it possible to produce GO nanocomposites in the form of fibers, films, conductive inks, electrodes and the like. The graphene oxide in these structures is dispersed in a polymer matrix. However, these materials do not exhibit advantageous electrical properties as graphene oxide is an electrical insulator. Thus, the polymer composites containing graphene oxide are not generally used for applications which require a high conductivity or permittivity. It is thus advisable to reduce the graphene oxide in order to form reduced graphene oxide (rGO), which exhibits the property of being conductive. One of the methods known for reducing graphene oxide in a nanocomposite consists in heating the nanocomposite. The efficacy of the reduction increases in proportion as the treatment temperature increases. However, the temperature potentially applicable is limited by a possible decomposition of the polymer matrix. This is because, if an excessively high temperature is used, the graphene oxide will be effectively reduced but the polymer matrix will be decomposed. It is thus advisable to choose a moderate temperature, which generally does not make it possible to obtain a material exhibiting good electrical properties. Consequently, the reduction of the graphene oxide is not very effective and the electrical and dielectric properties of the material obtained are mediocre.
It is known to use “ex situ” reduction methods which include a reduction of the graphene oxide before forming a composite with a polymer. Numerous methods are known for reducing graphene oxide. There thus exist chemical methods based on the use of hydrazine, electrochemical methods, thermal methods with high temperatures and also processes using water under supercritical conditions (SCW for supercritical water).
The publication entitled “Hydrothermal Dehydration for the “Green” Reduction of Exfoliated Graphene Oxide to Graphene and Demonstration of Tunable Optical Limiting Properties” by Yong Zhou and al., Chemistry of Materials, 2009, 21(13), 2950-2956, discloses a hydrothermal dehydration method for converting graphene oxide into a stable reduced graphene oxide (rGO) solution.
The chemical methods consist, for example, in employing reducing agents in a solution of particles of latex and of GO in order to reduce the graphene oxide. The drying of such solutions makes it possible to obtain a polymer-rGO composite. However, such a method requires compatibility between the reducing agents and the polymer.
The patent application EP 2 678 266 describes a method for the production of polymer nanocomposites starting from a dispersion of graphene oxide in a polyurethane latex, followed by a chemical reduction in order to form sheets of reduced graphene oxide. This treatment is rather burdensome since the polymer-reduced GO mixture has to be treated so as to obtain a polymer/rGO composite material.